PYONGYANG - Impoverished North Korea rejected international protests over its planned long-range rocket launch and said on Wednesday that it was injecting fuel "as we speak", meaning it could blast off as early as Thursday.

If all goes to plan, the launch, which North Korea's neighbors and the West say is a disguised ballistic missile test, will take a three-stage rocket over a sea separating the Korean peninsula from China before releasing a satellite into orbit when the third stage fires over waters near the Philippines.

Regional powers also worry it could be the prelude to another nuclear test, a pattern the hermit state set in 2009.

"We don't really care about the opinions from the outside. This is critical in order to develop our national economy," said Paek Chang-ho, head of the satellite control centre at the Korean Committee of Space Technology.

Once the refueling has been completed, the North Koreans will have to inject chemicals into the rocket which cause corrosion, which means the firing could come on Thursday, at the start of a five-day window announced already by Pyongyang.

Weather conditions on the peninsula also appear to favor a launch on Thursday or Saturday, according to meteorological reports from Japanese television.

"The likelihood of a launch (on Thursday) is the greatest," said Francis Yoon, a professor of engineering at South Korea's Yonsei University and an expert on rocket technology.

The launch of the Unha-3 rocket, which North Korea says will merely put a weather satellite into space, breaches UN sanctions imposed to prevent Pyongyang from developing a missile that could carry a nuclear warhead.

James Oberg, a former rocket scientist with the US space shuttle mission control who is in North Korea, said the rocket was not a weapon, but "98 percent of a weapon", requiring more technology, although not much.

This is the third long-range rocket test by North Korea. It says its second succeeded in putting a satellite into orbit in 2009, although independent experts say it failed.

The firing coincides with the 100th birthday celebrations of the founder of North Korea, Kim Il-sung, whose young, untested grandson, Kim Jong-un, now rules. Kim Il-sung died in 1994.

At a national conference of the ruling Workers' Party, Kim Jong-un was named first secretary, a new post created to give him the official stature to head the state where his grandfather remains "eternal president."

His father was also named party general secretary for eternity at the conference, the North's KCNA news agency said.

Paek, briefing foreign journalists in the North Korean capital of Pyongyang, declined to comment on the launch date.

"As for the exact timing of the launch, it will be decided by my superiors", Paek said.

South Korea, which remains technically at war with the North after their 1950-53 conflict ended with a truce rather than a peace treaty, warned Pyongyang it would deepen its isolation if it went ahead with the launch.

Security sources in Seoul, citing satellite images, have said that North Korea, which walked out of "six-party" disarmament talks three years ago, is also preparing a third nuclear test following the launch, something it did in 2009 and a move bound to trigger further condemnation and isolation.

South Korea holds parliamentary elections on Wednesday, although the rocket does not appear to have been a major issue with voters more concerned about job security.

US Secretary of State Hillary Clinton warned that history pointed to "additional provocations" from North Korea after the launch, an apparent reference to a nuclear test.

"This launch will give credence to the view that North Korean leaders see improved relations with the outside world as a threat to their system," she told cadets at the US Naval Academy in Annapolis, Maryland.

"And recent history strongly suggests that additional provocations may follow."

She also called on China to do more to ensure regional stability.

China, impoverished North Korea's only major ally, on Wednesday reiterated its pleas for calm and said all sides should make efforts to establish peace in the region.